Pneumocystis carinii pneumonia, caused by a fungus, is the most common lethal infection of patients with acquired immunodeficiency syndrome (AIDS). We have crystallized and solved the structure of the enzyme thymidylate synthase from this organism and plan to take a structure based approach to designing species specific drugs against this target. The crystals are thin rods (.01mm x .05mm x 1mm) and require synchrotron radiation to acquire high resolution data. In 1993 a complete data set was obtained at SSRL to 2.6 w resolution using a crystal which was frozen at -180lC. We wish to obtain even higher resolution data on slightly larger crystals at SSRL.